The manufacture of integrated circuits involves forming windows on the substrate for locating the positions of elements or for treatment. This substrate is coated with a layer of photosensitive resin. The windows are formed by masking this resin with a mask. Previously, direct contact or proximity was used as transfer method. Present processes use the technique of transfer by optical projection.
This projection may be effected with a 1/1 ratio and the mask is projected as a whole onto the wafer. This projection may also be effected by dividing the image either through analysis of the mask by means of a mobile slit, or by using the 1/n ratio photo-repetition technique.
In the technique of circuit manufacture by direct photo-repetition, each pattern is projected directly onto the semiconductor wafer previously coated with a layer of photosensitive resin, in accordance with a preestablished programme, the position of the wafer being controlled by interferometry in each direction. Conventionally, photo-repetition is carried out by shifting the wafer in the two orthogonal directions x and y. During the first exposure of the wafer, there are no particular difficulties for positioning, only that the successively exposed zones must be parallel to the plane of the pattern to be projected. During subsequent exposures, on the contrary, it is absolutely necessary for the projected patterns to be centered and aligned in relation to the previously projected patterns, for all zones of the water.
In fact, during projection of another pattern for providing another integration level of this circuit, the wafer must be placed in the same position as during the first exposure, so that there is correct superimposition of the different patterns. For this, it is necessary to center the wafer and to cause it to undergo a rotation which brings it into its first position. Thus, if the second pattern to be projected is exactly positioned in relation to the previously projected pattern, there will be coincidence.
In the case of an overall transfer, the alignment required for proper superimposition of the images is achieved as a whole by using two reference marks on each image plane, diametrically opposite each other for maximum accuracy. In the case of direct photo-repetitions, overall alignment is required for defining the angular position of the wafer and this alignment may be sufficient if the distribution of the future semiconductor chips on the wafer corresponds to a known linear law. However, if maximum accuracy is desired, periodic realignment, even chip by chip, may be necessary.
The invention may be applied to all the cases of transfer by projection: overall, by scanning or photo-repetition. However it is in this last case that it presents most interest.
In the prior art, the most widespread method consists in superimposing reference patterns plotted on the mask and on the semiconductor wafer by means of optical transfer means and in observing this superimposition with a two-lens microscope of the so-called "Split Field" type. The eye of the operator may be advantageously replaced by a vidicon tube for display on a television screen.
According to a more sophisticated technique, the video signal obtained by the television camera may be processed to obtain an analog or digital signal, usable by servo-control circuits. Some processes use for example St. Andrew's cross scanning and its complement for determining the deviation at alignment which results in an interval between the leading edges of the signals.
The devices using the processes of the prior art have two major disadvantages:
the field of the reference patterns plotted on the mask and the semiconductor wafer is completely illuminated and, on the images of these references is superimposed a not inconsiderable noise level (parasite reflection, diffusion, diffraction) which leads to a poor signal/noise ratio;
the contrast of the silicon reference marks is very variable from one integration level to the other, taking into account the variations in oxide thickness or the nature of the deposits (polycrystalline silicon, aluminium). The quality of the alignment will then be essentially variable. It depends either on the keenness of sight of the operator, or on the power of resolution of the associated electronic means.